Automobile control systems are operated by electrical circuitry and sensors. For example, the powertrain control system receives inputs from several sensors that are processed in an electronic powertrain control module. The sensors and the circuitry in the powertrain control module are powered from a reference voltage that has a lower voltage than the battery voltage, for example, five volts.
The powertrain control module includes a microcontroller running an application program that controls the powertrain in response to the sensor inputs. The application program typically includes a diagnostic that is useful in detecting the failure of various components in the powertrain control system. Upon detecting a failure, the diagnostic stores one or more diagnostic error codes in a nonvolatile memory. The diagnostic error codes can later be read by a service technician and used as a basis for effecting repairs by replacing the faulty component or components.
A problem occurs when the failure is due to a loss of the reference voltage. Known reference voltage diagnostics compare the reference voltage to fixed high and low threshold values. In order to detect one possible reference voltage fault, in which the reference voltage line gets shorted to the higher battery voltage, it is necessary to scale the reference voltage to a lower value. This scaling can be accomplished by a resistor divider. However, the resistance of the resistors varies significantly over time as the temperature of the engine compartment varies. The circuit that samples the scaled value also suffers from errors and the reference voltage itself fluctuates over time. Thus, to accommodate all normal operating conditions over the life of the vehicle, the thresholds must be set to a wide range to avoid detecting false failures.
However, using these wide thresholds causes problems when the diagnostic does not detect real reference voltage failures quickly enough. If the reference voltage fails, other components may appear to be failing when the only reason they are failing is that their power supply voltage has failed. If the portion of the diagnostic that detects reference voltage failure is not sensitive enough, the failure may not be detected before the failure of other components is recorded. Thus, the service technician who reads the diagnostic error codes will be directed to the wrong problem, resulting in unnecessary service work, which is expensive and time consuming for the owner.
Accordingly, it is desirable to provide a new diagnostic that is able to detect a reference voltage failure more accurately and to report this failure by setting an appropriate diagnostic error code, while ignoring other consequential failures. This and other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.